Conventionally, it has been considered that, in order to improve mechanical properties, it is desirable to retain crystal grains to be fine in gears, bearing parts, shafts and other mechanical parts, and have been performed in that manner.
For example, in mechanical parts such as gears and bearing parts, which require high surface strength, JIS steel types such as SCr420 are generally used after having been processed into the shapes of parts and then subjected to a surface-hardening treatment by carburization hardening. However, in this case, studies for refining crystal grains as much as possible have been conducted conventionally.
Specifically, when the above-described parts are subjected to a carburizing treatment, particularly, when the parts are subjected to a carburizing treatment at a high temperature, coarsening of the crystal grains in the surface layer is easily caused.
Conventionally, various studies and proposals to prevent crystal grains from becoming coarse in a surface layer have been made.
A technique of pinning grain boundaries by precipitating nitride particles such as AlN and Nb (C,N) in a dispersed state as pinning particles at a manufacturing step before a carburizing treatment has been widely known as a technique for preventing crystal grains from becoming coarse, and techniques of this kind are disclosed in, for example, Patent Document 1 and Patent Document 2 below.
In the technique of precipitating nitride particles of AlN or the like in a dispersed state as pinning particles at a manufacturing step before a carburizing treatment, in order to sufficiently precipitate the nitride particles of AlN or the like (pinning particles), large amounts of N and Al or Nb are added in steel in advance.
In this case, coarsening of the crystal grains is prevented in the surface layer at the time of the carburizing treatment, and crystal grains in the inner portion of the part are also retained to be fine due to the pinning effect by nitride particles of AlN or the like to be precipitated in the steel.
It has been conventionally considered that a part in which the crystal grains in the surface layer are prevented from becoming coarse and the crystal grains in the inner portion of the part are refined in this manner, that is, a part having fine crystal grains in both of the surface layer and the inner portion of the part, has satisfactory mechanical properties, particularly, satisfactory fatigue properties.
In addition, in Patent Document 3 below, in order to prevent denitrification and coarsening of crystal grains caused by the denitrification during a carburizing treatment, or in order to ensure fine crystal grain stability by supplying nitrogen to a surface layer to form nitride particles on the surface layer, a technique of introducing a nitriding gas such as NH3 into a treatment furnace during a vacuum carburizing treatment (including a temperature rising period by heating) is disclosed.
However, in the technique disclosed in Patent Document 3, the nitriding gas introduction is merely described and a relationship in quantity of nitride particles and a relationship in crystal grain size of nitride particles between the surface layer of the part and the inner portion of the part, and the like are not described.